1. Get a spreadsheet, excel will do, although increasingly I recommend Google docs spreadsheet because it's simpler to share with others.
2. Lay out along a single row a number of years. I would suggest ten years to start.
3. In the first year show the total investment required as a negative number (because the investors are sending their money to you).
4. In the first through tenth years, show the returns to the investors (after your share). This should be a positive number.
5. Then add those two rows together to get a "net cash flow" number.
6. Sum up the totals of all ten years to get total money in, total money back, and net profit.
7. Then calculate two numbers. The "multiple" is the total money back divided by the total money in. And then using the "IRR" function, calculate an annual return number.
Here's a link to Google docs where I've posted this example(http://spreadsheets.google.com/ccc?key=0AvqoGGDowi93dGZmZUp3RVFrZGpxRHZfazJwZWhmRlE&hl=en). It is public so everyone can play around with it and see how the formulas work.
It's worth looking for a minute at the theoretical example. The investors put in $400k, get $100k back for four years in a row (which gets them their money back), but then the business declines and eventually goes out of business in its seventh year. The annual rate of return or ROI(http://www.entrepreneur.com/encyclopedia/return-on-investment-roi )(return on investment) on the $400k turns out to be 14 percent and the total multiple is 1.3x.